Socially and professionally, people in modern society rely more and more on electrical devices. Video displays in particular are increasingly common elements of professional and personal spaces appearing in cell phones, automated checkout lines, banking systems, PDAs, and of course displays for desktop and laptop computers and HDTV systems.
Especially for display devices, but also for other electronic devices, typically a plurality of thin film devices are incorporated into such devices. For displays, one or more transistors are commonly used to control the behavior of each pixel within the display. The individual nature of each pixel of an LED, plasma, electrophoretic, or LCD display introduces the possibility that each pixel may provide a different quantity of light. One pixel may be brighter or darker than another, a difference that may be quite apparent to the viewer. Circuit components such as logic gates and interconnects are typically used to control the transistors and or other components.
As a flat screen display may incorporate millions of thin film devices, great care is generally applied in the fabrication of LED, plasma and LCD displays in an attempt to ensure that the pixels and their controlling circuits are as uniform and consistently alike as is possible. Frequently, especially with large displays, quality control measures discard a high percentage of displays before they are fully assembled. As such, displays are generally more expensive than they otherwise might be, as the manufacturers must recoup the costs for resources, time and precise tooling for both the acceptable displays and the unacceptable displays.
Traditionally, thin film devices have been formed through processes such as photolithography. In a photolithographic process, a substrate is provided and at least one material layer is uniformly deposited upon the substrate. A photo-resist layer, also commonly known as a photoresist, or even simply a resist, is deposited upon the material layer, typically by a spin coating machine. A mask is then placed over the photoresist and light, typically ultra-violet (UV) light, is applied through the mask to expose portions of the photoresist. During the process of exposure, the photoresist undergoes a chemical reaction. Generally, the photoresist will react in one of two ways.
With a positive photoresist, UV light changes the chemical structure of the photoresist so that it is soluble in a developer. What “shows” therefore goes, and the mask provides a copy of the patterns which are to remain—such as, for example, the trace lines of a circuit. Photolithography may also be considered a 2D process, in that each layer of material is deposited and then masked. Although 3D structure may be created by stacking layers patterned via the 2D process, there is no inherent alignment feature between the layers.
A negative photoresist behaves in the opposite manner—the UV exposure causes it to polymerize and not dissolve in the presence of a developer. As such, the mask is a photographic negative of the pattern to be left. Following the developing with either a negative or positive photoresist, blocks of photoresist remain. These blocks may be used to protect portions of the original material layer, or serve as isolators or other components.
Very commonly, these blocks serve as templates during an etching process, wherein the exposed portions of the material layer are removed, such as, for example, to establish a plurality of conductive rows.
The morphology of the materials composing each material layer, and specifically the crystalline texture of each material at an interface between materials is often of significant importance to the operation of the thin film device. Surface defects and surface contaminants may negatively affect the interfaces between layers and possibly degrade the performance of the thin film device.
In addition, photolithography is a precise process applied to small substrates. In part this is due to the high cost of the photo masks. For the fabrication of larger devices, typically rather than employing a larger and even more costly photo mask, a smaller mask is repeatedly used—a process that requires precise alignment.
As a photolithographic process typically involves multiple applications of materials, repeated masking and etching, issues of alignment between the thin film layers is of high importance. A photolithographic process is not well suited for formation of thin film devices on flexible substrates, where expansion, contraction or compression of the substrate may result in significant misalignment between material layers, thereby leading to inoperable thin film devices. In addition a flexible substrate is not flat—it is difficult to hold flat during the exposure process and thickness and surface roughness typically can not be controlled as well as they can for glass or other non-flexible substrates.
The issue of flatness in photolithography can be problematic because the minimum feature size that can be produced by a given imaging system is proportional to the wavelength of the illumination divided by the numerical aperture of the imaging system. However the depth of field of the imaging system is proportional to the wavelength of the illumination divided by the square of the numerical aperture. Therefore as resolution is increased the flatness of the substrate quickly becomes the critical issue.
With respect to the flat screen displays introduced above, use of flexible substrates for the internal backplane controlling the pixels is often desired. Such a flexible substrate can provide a display with flexible characteristics and significant weight reduction for mobile applications. A flexible substrate may also be easier to handle during fabrication and provide a more mechanically robust display for the user.
In addition, many thin film devices involve components that rely on crossovers, as in one conductor crossing over another conductor, or the isolation of one or more internal layers from other layers. One type of fabrication method that has been advancing is roll-to-roll processing. Roll-to-roll processing provides continuous steady state processing with high throughput. In addition, as the imprinting template used to define the desired thin film structures is a continuous pattern provided by cylinder, in most instances roll-to-roll systems can be provided in smaller physical spaces, thereby permitting smaller clean room environments and reduced equipment costs. As roll-to-roll processing involves a flexible substrate, the alignment of features and establishing crossover isolation can be somewhat challenging.
The mask used in a roll-to-roll process is typically provided by an imprint polymer. Deposited in liquid form and then imprinted to define the desired structure, an issue that frequently arises is that of fluid flow. More specifically, the liquid imprint polymer must flow into some areas of the imprint template and away from other areas of the imprint template. To facilitate speed and reliability of repetitive imprinting it is desirable to limit the distance that the fluid must flow. If there is excess imprinting fluid in one or more areas the result is likely to be that some or all polymer layers will be thicker then expected. Conversely if there is insufficient imprint liquid in one or more areas the result is that some or all of the polymer layers will be thinner then expected. As the thickness or thinness of one or more polymer layers directly correlate to the resulting structures, errant instances of inappropriate thinness or thickness may adversely affect the resulting thin film device.
Hence, there is a need for a thin film device that overcomes one or more of the above identified shortcomings.